1. Field of the Invention
The present invention relates generally to image scanning apparatuses employed in copying apparatuses and the like and, more particularly, to an image scanning apparatus in which originals that are scanned in forward scanning of a forward and backward scanning system to be subjected to image exposure are illuminated by an exposure lamp which lights on each time the originals are scanned.
2. Description of the Related Art
An exposure lamp for illuminating originals generates a great amount of heat. Thus, when a large number of sheets are continuously copied as in recent years, a platen glass for supporting the originals to be scanned is heated dangerously up to a high temperature by the generation of heat from the exposure lamp. In order to suppress this rise in temperature, it has been conventionally structured, upon repetitive and continuous image scanning by a scanning system, that the exposure lamp once lights off each time the scanning is terminated and then lights on again upon the next scanning so as to carry out image exposure, resulting in a decrease in total time period during which the exposure lamp is lighting on in case where a large amount of copies are continuously made.
The timing at which the exposure lamp lights on again should be unobjectionable to the image exposure in view of the rising time of the lamp to a predetermined amount of light. In order to satisfy this requirement, alternative two methods have conventionally been adopted. The one is lighting on the exposure lamp again after a definite time period has passed since the scanning system completes scanning. The other method is lighting on the exposure lamp when a position sensor provided at a fixed position detects the scanning system in backward scanning.
FIG. 13A is a schematic diagram showing movement of the scanning system in case where the scanning system starts scanning; and FIGS. 13B and 13C are schematic diagrams showing movement of the scanning system with each different size of copying in case where continuous scanning starts.
Referring to FIG. 13A, the scanning system exists at a predetermined position Sx before scanning. In response to a scanning instruction, the scanning system lights on the exposure lamp and moves toward a scanning start position Ss where the originals are to be scanned. When the scanning system passes through the scanning start position Ss, the exposure lamp provides a predetermined amount of light. With the scanning terminated, the exposure lamp lights off, so that the scanning system moves in the opposite direction to the scanning direction at a return position S.sub.1. When reaching a home position S.sub.0, the scanning system is inverted again in the scanning direction to carry out the next scanning. In this case, the exposure lamp is required to light on upon backward scanning of the scanning system so that the amount of light generated by the exposure lamp may reach a predetermined amount in the next scanning, i.e., at the scanning start position Ss in re-scanning.
It is, however, disadvantageous to light on the exposure lamp after a definite time period has passed since the scanning system completes scanning. The position S.sub.1 where the scanning system completes scanning and makes a return differs depending on the size of copying in such a case that scanning distances s1.sub.1 and s1.sub.2 are determined dependently on copying size z1 and z2 as shown in FIGS. 13B and 13C, simply in view of equal-scale magnification copying. Thus, if the exposure lamp again lights on at a position S.sub.2 after a definite time period t has passed since the termination of the scanning by the scanning system, a distance provided from the time point S.sub.2 when the exposure lamp again lights on to the time point when the scanning system returns to the home position S.sub.0 varies as rs.sub.1 and rs.sub.2 according to the copying size. Accordingly, a time period required for the scanning system to reach the scanning start position Ss after the exposure lamp lights on again becomes different as t.sub.1 and t.sub.2, thereby to affect the amount of light generated by the exposure lamp. Therefore, even in case of the minimal copying size z.sub.2 requiring the minimal time period t.sub.2, the re-light-on timing of the exposure lamp need be set so as to obtain a necessary rising time for the exposure lamp upon re-lighting on. In case of the copying size z.sub.1 larger than the minimal copying size z.sub.2, however, the exposure lamp rises earlier to a predetermined amount of light by a time t.sub.3, that is, by the difference between the minimal copying size z.sub.2 and the larger copying size z.sub.1, and hence the platen glass is heated excessively by extra temperature corresponding to the time t.sub.3, leading to vain power consumption.
For the above-described reasons, such a method is considered that the exposure lamp again lights on when the sensor provided at a fixed position detects the scanning system under backward scanning, without being affected by the copying size.
FIGS. 14A and 14B are schematic diagrams showing the movement of the scanning system when this method is adopted. Referring to the figures, even if the scanning distance of the scanning system varies as s1.sub.1 and s1.sub.2 depending on the copying size z.sub.1 and z.sub.2, the exposure lamp can light on again at the position S.sub.2 where a distance required for the scanning system to return to the home position S.sub.0 becomes rs in common because the sensor se is at a fixed position. This eliminates such inconveniences as given in the above-described conventional example.
However, the scanning system moves at different scanning speed sv depending on copying magnification. When circumferential speed (system speed) of a photoreceptor to be subjected to the image exposure by the scanning system is represented by v, an equation sv=v/n (n: copying magnification) is obtained. The scanning speed is 2v in a contraction where magnification n is 1/2, whereas it is v/2 in an enlargement where magnification n is 2. Accordingly, as shown in FIGS. 13A and 13B, even if the exposure lamp again lights on at the position S.sub.2 where the scanning system gains a definite distance rs from home position S.sub.0, a time period ts.sub.1, ts.sub.2 required at least when the scanning system returns to home position S.sub.0, then moves forward for the subsequent scanning and reaches the scanning start position Ss becomes ts.sub.1 .noteq.ts.sub.2 in FIGS. 14A and 14B because of different scanning speed, provided that there is a difference in copying magnification processing between FIGS. 13A and 13B. Consequently, there is no other way then setting the re-light-on timing of the exposure lamp by the sensor se so as to obtain a necessary rising time for the exposure lamp upon re-lighting on in case of the minimal magnification corresponding to the minimal required time, e.g., ts.sub.1. In addition, in case of a magnification larger than the minimal magnification, the exposure lamp rises earlier to a predetermined amount of light by the time corresponding to the difference in magnification, and hence the platen glass is heated excessively and the power is vainly consumed.
As described above, the conventional canning apparatus is still disadvantageous with respect to the extra heating of the platen glass and the vain power consumption. Meanwhile, the backward scanning speed of the scanning system is increasingly enhanced for achieving still higher speed of operation, resulting in a decreased opportunity for the exposure lamp to light off. Therefore, it is indispensable to avoid the extra increase in temperature and vain power consumption.